When analysed by frontier molecular orbital (FMO) topology, it transpired that there are four general types of Lewis base:

  • s-HOMO Lewis Bases

  • Complex Anion Lewis Bases

  • Lobe-HOMO Lewis Bases

  • π-System Lewis Bases

s-HOMO Lewis Bases



Hydride Ion, Hydrogen & Helium

  • H D T

  • H2 D2 T2

  • HD HT DT

  • He

FMO Topology:

s-HOMO Lewis bases have spherical 1s or ovoid (peanut) 1s HOMOs which are devoid of the closed electron shells which hinder complexation (due to closed-shell/closed-shell repulsion) seen in all other Lewis base types.



Negative (H, hydride) or neutral (H2, hydrogen).


Intrinsically soft. The softest and most polarisable of all species.



Nearly all of the elements are able to form hydride compounds:


When s-HOMO Lewis bases interact with Lewis acids to form a complex they are deemed to reduce the Lewis acid: s-HOMO Lewis bases are reducing agents.

The hydride ion is a strong proton abstracting base and nucleophile. H2 is only very weakly basic such that H2, D2, T2, HD, HT and DT hardly seem to be basic at all, yet as they can be protonated to [H3]+ (in the gas phase) they must be Lewis bases.

H and H2 form metallic complexes.

Helium can be protonated under gas phase conditions to [HHe]+ so it is a Lewis base, an s-HOMO Lews base.

Congeneric Series:

  • H D T

  • H2 D2 T2

  • HD HT DT

  • He

Complex Anion Lewis Bases


High Symmetry Molecular Anions


Search for complex anion Lewis bases species in The Chemical Thesaurus

FMO Topology:

Complex anion Lewis bases have a hypervalent central cation (boron, aluminium or heavy metal) saturated with anionic Lewis base ligands. Lewis octet and 18-electron rules are generally satisfied. The HOMO shows high spherical symmetry.




Intrinsically hard.


Complex anion Lewis base species behave as charged hard spheres that form ionic charge-controlled complexes (ie act as non-nucleophilic counter ions), or they behave as donors of hard/soft ligands, X.

Ligand substitution – in which a nucleophilic Lewis Base displaces a nucleofugal ligand – is common and ligand symbiosis considerations/effects are very important.

There are four subclasses of complex anion Lewis base:

X = Halogen anion which gives rise to the synthetically useful non-basic, non-nucleophilic, non-interfering anionic spectator counter ions:

  • [BF4]
  • [SbF6]
  • [AsF6]
  • [FeBr4]

X = Hydride ion which gives rise to species which act as donors of nucleophilic hydride ion, [BH4] and [AlH4], as long as there is not a Brønsted Acidic proton available or H2 is generated.

M = Heavy metal (Fe or Cr as opposed to B or Al). Such complex anions are much studied in classical inorganic coordination chemistry. Transition metals centres often exhibit multiple oxidation states. These are better considered as type 23 Lewis acid/base complexes.

X = Oxygen heavy metal species with oxygen ligands are commonly used as oxidising agents.

Congeneric Series:

Families of ligand replacement congeneric series are common:


Lobe-HOMO Lewis Bases


Electron Lone Pair Species


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FMO Topology:

Typical electron lone pair species in which the Lewis base HOMO is a px FMO orbital.

In valence bond terms, the electron pair is in an sp3, sp2 or sp hybrid orbital.

On complexation species show directional bonding which implies that the HOMO is directional, ie lobe shaped.


Negative or neutral with a lone-pair of electrons.


Species range from hard to soft. The hardness of anionic Lobe-HOMO Lewis bases can be defined with respect to the methyl anion, H3C the carbon-hydrogen bond length in methane (109pm).

All Lobe-HOMO Lewis bases can by definition be protonated and the conjugate Brønsted acid's proton-to-Lewis base bond length, along with its pKa value, serves to probe the Lewis base's chemistry and behaviour. Bond-length and pKa data are linear over congeneric series and planars.

Methane, CH4, is a convenient reference Lobe-HOMO Lewis base due to the importance of carbon in organic chemistry and that so many reactions occur at carbon centres.

Congeneric species with base-to-H+ bond lengths shorter than 109pm, such as the hydroxide ion HO (HO–H bond length = 96pm), are deemed to be harder than the methyl carbanion. Longer bond-length as seen with iodide, I, (HI bond length = 161pm) equates with softness.


Lobe-HOMO Lewis bases are the classic electron lone-pair donor species that can act as:

  • Proton abstracting Brønsted bases
  • Ligands around metal complex ions
  • Attacking nucleophiles
  • Nucleofugal leaving groups
  • Non-interfering spectator anions
  • Co-complexing or solvating agents

Lewis acid/base complexes formed by Lobe-HOMO Lewis bases range from FMO controlled covalent carbon-carbon bonds to charge-controlled ionic species such as cesium fluoride, CsF.

The atomic lone pair centre may be embedded in a π-system, for example the allyl ion, in which case the species can be dual classified as a Lobe-HOMO and a π-HOMO Lewis base. When the species reacts via a single atomic centre it is classified as a Lobe-HOMO Lewis base.

There are several subclasses of Lobe-HOMO Lewis bases, including:

  • Carbanion centres: H3C, enolates, etc., are of huge importance in organic chemistry because the carbanion can act as a nucleophile at a carbon-nulceofuge centre, C-Nfg, such as an alkyl bromide or tosylate. The effect is to increase the length of the carbon skeleton. The rule is that π-stabilisation makes a carbanion centre less basic but more nucleophilic.
  • α-Effect ("alpha effect") bases like hydrazine and hydrogen peroxide are rendered more basic & more nucleophilic by an adjacent lone-pair of electrons. Find α-effect ("alpha effect") Lewis bases in The Chemical Thesaurus
  • Bidentate and polydentate bases have two or more similar Lewis base centres and are commonly employed as ligands in transition metal chemistry. Find bidentate and polydentate bases in The Chemical Thesaurus
  • Ambident (ambidentate) bases have two dissimilar Lewis base centres and show selectivity. Find ambident and polydentate bases in The Chemical Thesaurus.

    Congeneric Series:

    There are two important congeneric planars to be found within electronegative main group elements and their anions. One is formed by the X anions and the other by X: neutral lone pair species.

    Read more about the chemistry of these congeneric planars elsewhere in this webbook:


    The "Inerts"

    A number of Group 14 elemental hydrides: CH4, SiH4, GeH4 & SnH4, are rather inert towards Lewis acid and Lewis base reagents. (Species can be oxidised and they are susceptible to attack by radicals and diradicals.)

    However, methane can be protonated by super acids to the carbonium ion:

    H+ + CH4 –> [CH5]+

    So methane is a Lewis base but, like helium, it is an exceedingly feeble proton abstractor. It follows that: CH4, SiH4, GeH4, SnH4 and related compounds are Lewis bases.

    Search for inert species in The Chemical Thesaurus

    π-System Lewis Bases


    Electron Rich π-Systems


    If an extended electron rich π-system species reacts via a single atomic centre, for example when an allyl anion is protonated to give propene, the species is better considered behaving as an ambidentate, π-stabilised Lobe-HOMO Lewis base.

    However, when when the species reacts via its extended π-system directly, for example during Diels-Alder cycloaddition or when forming a π-organometallic complex, the the species should be considered as a π-HOMO Lewis base species. Thus, there is an overlap between π-stabilised Lobe-HOMO and π-HOMO classification.

    Search for π-HOMO Lewis base species in The Chemical Thesaurus

    FMO Topology:

    π-HOMO Lewis bases have their electrons in their highest occupied molecular orbital, or HOMO, delocalised over two or more p-orbitals.

    Species require modelling by Hückel-FMO techniques as well as by valence bond (VB)-resonance structure methods. Hückel MO modelling gives rise to whole families of π-structure: polyene ribbons, aromatics, etc.:


    Indeed, quantum mechanics is all about patterns. A particularly striking manifestation is seen with the polyene system of: 1, 2, 3, 4, 5, 6... conjugated p-orbital systems and how they give rise to the carbanion, allyl anion & pentatrieneyl anion and alkene, 1,3-diene & 1,3,5-triene π-HOMO Lewis bases:



    Negative, delta or electron rich π-systems.


    Soft when the entire π-system is acting as the Lewis base, but harder when a single atomic centre is involved and the species is behaving as a Lobe-HOMO Lewis base.

    The allyl anion can behave as (or be considered as) 2 π-electrons delocalised over a 3p orbital function, or as a stabilised 2p orbital carbanion. In this latter case it is better to consider the allyl anion to be behaving as a (harder) Lobe-HOMO Lewis base.


    Species behave as π-species when they undergo FMO controlled multicentre interactions. These most obviously manifest themselves in three situations:

    • Stabilisation of the π-system: certain patterns/structures are associated with stability such as 4n+2 π-electrons in a cyclic array, the allyl anion, etc. 
    • Diels-Alder cycloaddition and other pericyclic interactions, Type 20 Lewis acid/base complexation.
    • Formation of π-organometallic complexes, Type 24 Lewis acid/base complexation.

    Congeneric Series: